1. Field of Invention
This invention relates to the field of testing internal combustion engine fuel pumps and, in particular, the field of testing fuel pumps while the pump remains mounted on and driven by the engine to which fuel is supplied by the pump.
2. Discussion of the Prior Art
Engine fuel pumps are normally tested "off engine" whereby the fuel pump is removed from the engine and mounted on a special test stand. One example of such test apparatus is disclosed in U.S. Pat. No. 3,340,728 wherein the test stand includes a variable speed drive for controlling the pump speed during the test combined with gauges for measuring the flow characteristics of the pump. While suitable for some purposes, a far preferable approach has been the more recent development of various techniques by which a fuel pump may be tested "on engine", that is, while the pump remains mounted on and driven by the engine to which the pump normally supplies fuel. One example of the more desirable "on engine" fuel pump test devices is disclosed in U.S. Pat. No. 3,745,818 which discloses a portable apparatus for determining if adjustments are required in the calibration of a fuel pump by measuring fuel flow rates and pressures at selected engine speeds while the fuel pump remains mounted on the engine. Apparatus of the type disclosed in U.S. Pat. No. 3,745,818 can significantly reduce the time necessary for properly diagnosing and calibrating a fuel pump by eliminating the time required for transferring the pump from the engine to a test stand and back to the engine which procedure may easily require more time than does the actual fuel pump test.
While an "on engine" fuel pump test apparatus is well suited for many types of engines, even engines equipped with sophisticated fuel injection systems, special and unique problems arise when "on engine" fuel pump testing is performed on an internal combustion engine whose speed is controlled by the fuel pressure supplied by the pump to the engine. One example of such an engine is disclosed in U.S. Pat. No. 3,128,750. Manufacturer's specifications for calibrating fuel pumps often require the pump to produce a specified output pressure when operated under one of a plurality of check point conditions each of which is defined by a specific fuel pump speed and a corresponding output flow rate. In order to calibrate such a fuel pump while on the engine, some means other than the normal engine throttle must be provided to control selectively the engine speed thereby to cause the engine to drive the fuel pump at each of the check point speeds while the necessary flow rate and pressure measurements are made.
U.S. Pat. No. 3,577,776, assigned to the same assignee as the present application, discloses a fuel pump testing method and apparatus for an engine whose speed is responsive to fuel pressures wherein engine speed is controlled during an "one engine" fuel pump test by providing an auxiliary source of pressurized fuel. This source is completely separate from the engine's normal fuel supply system and includes an auxiliary fuel pump driven by its own motor and a manually operated control valve for selectively supplying a controlled fuel pressure to the engine whereby the engine speed may be independently controlled. Because an entirely separate fuel supply system is required, the apparatus disclosed in U.S. Pat. No. 3,577,776 tends to be expensive, heavy and complicated thus obviating many of the advantages normally associated with "on engine" fuel pump testing. The need for an auxiliary drive as well as an auxiliary fuel pump in the system of U.S. Pat. No. 3,577,776 virtually destroys the advantage of eliminating the test stand associated with "on engine" testing. Moreover the accuracy of any fuel pump test which involves operating the fuel pump at a constant check point speed will, obviously, depend on the degree to which the speed can be maintained constant. In order to achieve a high degreee of constancy in the selected speed of an engine responsive to fuel pressure, it would be necessary to employ hydraulic feedback to the control valve or to employ a variable speed auxiliary fuel pump drive combined with a pressure sensitive feedback control for maintaining extremely accurate control over the fuel pressure supplied to the engine during the pump test. In the absence of such accurate control it is difficult to operate an engine at a perfectly constant selected speed during the fuel pump test. Unfortunately, apparatus capable of providing such accurate fuel pressure control is expensive and complicated which adds greatly to the disadvantage of using a completely separate auxiliary fuel supply sustem. Still another disadvantage of employing an entirely separate fuel supply system is that the saftey equipment normally associated with the engine fuel system, such as the overspeed governor, is no longer operative when the engine is supplied with fuel from the auxiliary fuel system. Thus, safe operation of test apparatus involving a completely separate fuel supply system further requires a safety cut-off such as a fuel cut-off valve controlled by an overspeed or an overpressure sensor. Without such a safety device, an engine operated by such an auxiliary fuel supply could dangerously overspeed upon malfunction of the pressure regulator.